1. Field of the Invention
The present invention relates to improvements in serial boundary-scan testing of removable circuit boards located on a backplane bus. Specifically, the invention involves a method and apparatus for allowing each removable circuit board to be daisy chained together and connected to a common serial data return line to facilitate boundary-scan testing of circuit boards selectively installed on the bus.
2. Description of Related Art
The increasing complexity of circuit designs and new integrated circuit packaging techniques leads to an ever increasing problem of testing integrated circuits and printed circuit boards (pc-boards). Software testing of digital hardware is constantly improving, but the complexity of the circuitry makes testing much more difficult unless the components are designed with testability in mind.
One method for testing circuitry which has received notable attention is the IEEE 1149.1 boundary-scan standard originated by the international Joint Test Action Group (JTAG). One implementation of this standard involves designing components for serial boundary-scan testing by providing shift-register elements daisy chained to form a path around the periphery of an integrated circuit component. The idea behind the serial testing is to shift serial data into and through a number of circuit components. The serial data follows a known path and exits the circuits altered by the circuit functions. A master testing circuit compares the return data to an expected result dependent upon the effect the circuit should have on the data shifted into the circuit. In other words, the circuit being tested, if it is functioning properly, produces known changes in the data stream before the data exits the circuit. If the data stream returned to the master testing circuit is not as expected, then a malfunction in the circuit is detected by the testing circuit. Usually, a careful analysis (under software control) of the deviations in the data stream isolates any malfunctions within a circuit.
Boundary-scan testing of components, as briefly explained above, can also be expanded to testing of an entire pc-board. Preferably, all of the integrated circuits on the pc-board to be tested are designed according to the JTAG standard. Testing of the pc-board is carried out in a manner similar to testing of the individual components except that the serial data propagates from integrated circuit to integrated circuit serially through the pc-board and is returned to the master testing circuit from the very last JTAG component on the pc-board.
This testing method functions smoothly as long as no testable integrated circuits in the daisy-chain are removed from the pc-board tested. For instance, this testing scheme works well for testing a microprocessor motherboard because the functions remain unchanged, and the circuitry is not usually removed.
However, the daisy-chain scheme is complicated if the testing method is expanded to test several removable pc-boards installed in a backplane bus of a computer system unless each slot on the bus has a separate serial data input line and a separate data return line for individual testing of each pc-board. If the pc-boards in the slots are daisy chained for serial testing with only one serial data input line to the first board and one common data return line for all the potential circuit boards along the bus, then a number of problems arise.
The main problem involves the common return line. If more than one removable circuit board is installed on the bus, then the data exiting each board during testing operations causes data collisions on the return line. If only one return line is provided from the last slot on the bus, then if any slot is empty, the daisy-chain path for serial test data is broken and the data exiting the circuit boards never reaches the return line.
Therefore, it is desirable to provide a system for daisy-chain serial testing of removable circuit boards selectively installed along a backplane bus.